In recent years, sophisticated various electronic devices such as mobile phones, digital cameras, navigators and others have had smaller size and lighter weight, and wiring patterns formed on substrates have had finer pitch. In association therewith, as an electronic wiring material for these uses, substrates for flexible printed wiring (FPC substrate) are demanded to have higher performances, specifically, light transmission property as high as that provided by glass and adhesion with a wiring portion, as well as heat resistance and flexibility.
As a polymer for forming the flexible printed wiring board, for example, an all-aromatic polyimide, as shown in Patent Document 1, has been used (e.g., APICAL manufactured by Kaneka Corporation).
However, the substrate comprising the polyimide has yellow-brown coloration as a result of the formation of a charge transfer complex within a molecule and between molecules, and thus is difficult to use for an application requiring light transmission property as high as that provided by glass.
Furthermore, the formation of a film from the polyimide requires imidization at a temperature of from 300 to 500° C.